This invention relates to an optical system, optical unit or optical instrument with a diffractive optical element.
Many proposals have recently been made with respect to an optical system with a diffractive optical element, such as, for example, a Fresnel zone plate, a diffraction grating or a hologram.
A diffractive optical element may be used as an optical element for transforming a received wave front into a predetermined wave front. Such a diffractive optical element has unique characteristics not provided by a refraction type lens. For example, it has a dispersion value inverse to that of a refraction type lens, and also it has no substantial thickness. Thus, an optical system can be made compact.
For manufacture of a binary type diffractive optical element which is one of phase type diffraction gratings, a semiconductor device manufacturing technique is applicable to it, and a grating of fine pitch can be produced relatively easily. For this reason, many studies have been made directed to a binary type diffractive optical element with blazed shape being approximated by a step-like structure.
FIG. 1 shows a binary type diffractive optical element. This binary type diffractive optical element has a diffractive optical surface 204, having been formed by removing, from the shape of a flat-convex type refraction lens 201 (FIG. 1, (A)), portions that provide an optical path difference corresponding to n times the wavelength (n is an integer), and by quantizing the shape of a resultant diffractive optical element 202 (with a shape shown in FIG. 1, (B)) with a thickness corresponding to a fraction of the wavelength so that the shape is approximated with a step-like structure such as shown in FIG. 1, (C).
In FIG. 1, denoted at 203 and 205 each is a transparent substrate on which a diffractive optical surface 202 or 204 with a fine shape is formed.
FIG. 2 illustrates manufacturing processes for a known binary type diffractive optical element having a four-step (level) structure. Denoted in the drawing at 300 is a transparent glass substrate (refractivity: n), and denoted at 301 is a resist. Denoted at 302 is a mask to be used for a first exposure, and denoted at 303 or 306 is exposure light. Here, the resist 301 comprises a positive type resist.
First, in process A, the pattern of the mask 302 is transferred onto the resist 301 by means of exposure light 303. In process B, development of the resist 301 is performed and, in process C, etching of the glass substrate 300 is executed. Then, in process D, unnecessary resist material on the glass substrate 300 is removed, whereby a binary type diffractive optical element with a two-step (level) structure is completed.
Here, the depth d1 of etching is determined by:   d1  =      λ          2      ⁢              (                  n          -          1                )            
where xcex is the wavelength used with the binary type diffractive optical element.
Subsequently, a resist 304 is again applied to the glass substrate 300 having a binary type diffractive optical element with a two-level structure formed thereon, and in process E a second exposure is performed by use of a mask 305. The pattern of the mask 305 has a pitch a half of that of the pattern of the mask 302. The exposure is performed while accurately registering the edge of a light blocking portion of the mask 305 with the edge of the two-level binary structure. Then, after a developing process in process F, a resist pattern such as illustrated is provided.
Subsequently, in process G, a second etching process is performed and, in process H, unnecessary resist material is removed, whereby a binary type diffractive optical element with a four-step (level) structure is accomplished. Here, the depth d2 of etching defined by the second etching process is determined by:   d2  =      λ          4      ⁢              (                  n          -          1                )            
These processes are concerned with a four-level structure and, as is well known, by repeating these processes, a binary type diffractive optical element of an eight-level or a sixteen-level structure can be produced.
In the above-described method the number of steps (levels) that can be formed is limited to 2n (n is a natural number), but by selecting the number of masks used and the pattern line width suitably, a binary type diffractive optical element with a desired number of levels can be produced.
The approximation of a step-like shape may lead to a small decrease of diffraction efficiency. Practically, however, a diffraction efficiency of about 95% (with eight-step approximation) or about 99% (with sixteen-step approximation) will be attainable, and there will be no practical problem.
Many advantages will be provided, as described, with use of a diffractive optical element in a portion of an optical system. However, as regards a diffractive optical element, particularly, a binary type diffractive optical element, in many cases a parallel fiat plate is used as a substrate of the diffractive optical element. This is mainly because of ease of manufacture. Generally, the thickness of such substrate is small. Also, when a diffractive optical element is used in an optical system, particularly, in a projection optical system of a semiconductor device manufacturing apparatus such as a stepper, the effective diameter of the diffractive optical element may become very large to meet an increase of numerical aperture (NA). For a diffractive optical element of a large effective diameter and with a small thickness substrate, there is a possibility of deformation of the diffractive optical element due to its weight or deformation of the same when it is mounted in a barrel. It may cause degradation of imaging performance.
It is an object of the present invention to provide an optical system or an optical unit by which degradation of optical performance is reduced even when a diffractive optical element of large effective diameter and small thickness is used.
In accordance with an aspect of the present invention, there is provided an optical system, comprising: a diffractive optical element having a diffractive optical surface; and means for preventing a change in optical performance of said optical system due to deformation of said diffractive optical element produced when said diffractive optical element is provided in said optical system.
In accordance with another aspect of the present invention, there is provided an optical unit, comprising: a diffractive optical element having a diffractive optical surface; and a reinforcing member connected to said diffractive optical element substantially without damaging the function of said diffractive optical surface.
In accordance with a further aspect of the present invention, there is provided an optical instrument, comprising: an optical system, said optical system including (i) a diffractive optical element having a diffractive optical surface, and (ii) means for preventing a change in optical performance of said optical system due to deformation of said diffractive optical element produced when said diffractive optical element is provided in said optical system; and means for holding said optical system in said optical instrument.
In accordance with a still further aspect of the present invention, there is provided an optical instrument, comprising: an optical unit, said optical unit including (i) a diffractive optical element having a diffractive optical surface, and (ii) a reinforcing member connected to said diffractive optical element substantially without damaging the function of said diffractive optical surface; and means for holding said optical unit in said optical instrument.
In accordance with a yet further aspect of the present invention, there is provided a projection exposure apparatus, comprising: an illumination optical system for illuminating a pattern formed on a mask; and a projection optical system for projecting the pattern of the mask onto a wafer, said projection optical system including (i) a diffractive optical element having a diffractive optical surface, and (ii) means for preventing a change in optical performance of said projection optical system due to deformation of said diffractive optical element produced when said diffractive optical element is provided in said projection optical system
In accordance with a yet further aspect of the present invention, there is provided a projection exposure apparatus, comprising: an illumination optical system for illuminating a pattern formed on a mask; and a projection optical system for projecting the pattern of the mask onto a wafer, said projection optical system including (i) a diffractive optical element having a diffractive optical surface, and (ii) a reinforcing member connected to said diffractive optical element substantially without damaging the function of said diffractive optical surface.
In accordance with another aspect of the present invention, there is provided a device manufacturing method including a process for transferring, through projection exposure, a pattern of a mask onto a wafer by use of a projection exposure apparatus as recited above.
In accordance with a further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: transferring, through projection exposure, a pattern of a mask onto a wafer by use of a projection exposure apparatus as recited above; and developing the wafer having been exposed.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.